Conventionally, in an image display apparatus such as a head-mounted display (HMD) which is worn on the head of a user and displays images, various methods such as a method using a pixel-type display device including a liquid crystal device and an organic EL as an image display unit, and a method where the image is directly rendered on a retina of an eye by two-dimensional scanning with laser beam have been proposed.
In such an image display apparatus, the entire display apparatus need to be compact and light, in order to reduce the stress for the user upon wearing and to allow long-hour use. Furthermore, designing the image display apparatus similar to conventional eyeglasses allows constant wearing and activities just like when wearing the conventional eyeglasses.
However, the method using the pixel-type display device causes the display unit and ocular optical system using the prism and half mirror which guide the light generated on the display unit to the eye to be larger as the image quality and viewing angle increase, which makes it difficult to make them compact and light.
Furthermore, the ocular optical system covers the eyes, and this makes it difficult to implement a shape of conventional eyeglasses. Accordingly, the imaging apparatus appears more like goggles or helmets rather than eyeglasses. For this reason, the ocular optical system hardly fits naturally.
On the other hand, the retina-scanning display using the laser scanning method achieves an extremely small display device using a compact Micro-Electro-Mechanical System (MEMS) mirror device.
Furthermore, there is a proposal, in which thin optical system is achieved by using a holographic mirror for the ocular optical system instead of the prism and a half mirror, such that the entire apparatus takes a shape of eyeglasses (for example, see Patent Reference 1).
FIGS. 17A, 17B, and 17C show examples of the scanning image display apparatus 100.
FIG. 17A is a plan view, FIG. 17B is a side view, and FIG. 17C is a view from the eyes. Note that, each of the abovementioned diagrams shows only the left half of the head of the user and the scanning image display apparatus 100. However, when the scanning image display apparatus 100 is applicable to binocular vision, the structure is symmetrical on the left and right (the same applies to the description hereafter).
As shown in FIGS. 17A and 17B, the conventional scanning image display apparatus 100 includes a lens 110 which is arranged in front of the eyes of the user, and a temple 111 an end if which is connected to the outer rim of the lens 110 and the other end of which is fixed on the temporal region of the head of the user.
On the lens 110, the holographic mirror 104 is formed on the side facing the user's eye. On the temple 111, a light source 101 which emits laser beam, a biaxial scanning mirror 102 which two-dimensionally scans the holographic mirror 104 with laser beam, and a control unit 103 which controls each unit are incorporated.
The laser beam emitted from the light source 101 is projected on the lens 110 using the biaxial scanning mirror 102, reflects on the holographic mirror 104 formed on the lens 110, enters the eye 120 of the user, and forms an image on the retina. The holographic mirror 104 is, for example, a photopolymer layer on which Lippmann volume hologram is formed, and reflects only the wavelength of the laser beam by giving wavelength selectivity. As a result, the user can view both the outside scenery and images rendered by the laser beam at the same time.
In the scanning image display apparatus 100 with the structure described above, the optical axis of emitting the laser beam from the ear 121 side of the temple 111 (posterior to the biaxial scanning mirror when seen by the user) to an MEMS mirror and the central axis of the eye 120 are substantially parallel, when the MEMS mirror is used for the biaxial scanning mirror 102. Furthermore, the angle of incidence α (the angle formed with the normal line of the reflection surface and the axis of incident light) of the laser beam on the MEMS mirror is equal to the angle of incidence β from the biaxial scanning mirror 102 to the holographic mirror 104. More specifically, when the arrangement is made such that the laser beam from the MEMS mirror is projected on the holographic mirror 104 without being interrupted with the face of the user, α=β=approximately 60 degrees is satisfied.
In addition, there is an example where the structure is the same as the structure shown in FIGS. 17A and 17B, but the incident direction of the laser beam is different (for example, see Patent Reference 2).